1. Field of the Invention
The invention relates to a method for the manufacture of metal packaging cans by deep-drawing and ironing of metal sheet, and also relates to a semi-product (i.e. an intermediate product) in the manufacture of metal packaging cans.
2. Description of the Prior Art
In the packaging industry, so-called two-piece and three-piece cans are well known. In the case of a two-piece can the cylindrical body and the bottom form one seamless unity, which is obtained by deep-drawing and subsequent ironing of metal sheet. The second part is the lid or top. In a so-called three-piece can on the other hand it is possible to distinguish a cylindrical body, a bottom and a lid or top, the three components being as a rule joined to each other by flanges.
Where a two-piece or a three-piece metal packaging can is mentioned herein, this should be taken as a reference to both the still lidless and the filled and closed can, unless otherwise stated.
In the packaging industry, two-piece and three-piece cans, each in various versions, have found their own areas of application, these being determined by the specific requirements of the packed product or by other circumstances such as transport costs, suitability for printing, unit cost, etc.
While the two-piece drawn and ironed can has a seamless body, the body of the three-piece can has a seam. This body is conventionally manufactured by rolling a flat sheet and subsequently joining the ends thereof to form the seam by soldering, cementing or welding. French patent specification No. 1,211,354 shows a can having two seams formed by superposing two sheets and seaming their edges together. An elongate tube divisible into several such cans is shown.
With a view to cost saving, there is a tendency with three-piece cans to move towards thinner material for the body, for example to reduce the wall thickness of the body from a conventional value of about 0.16 mm towards the wall thickness usual in ironed cans of about 0.10 mm.
To provide a good grasp of the various terms used it should be noted that by "deep drawing" (German "Tiefziehen") is intended in general, and in this specification, a process by which sheet material is deformed geometrically without significant change in the thickness of this material. In contrast to this, "ironing" (German "Abstrecken") implies a process by which the material is drawn between a matrix and a die with simultaneous drastic reduction of the wall thickness. In a standard deep-drawn and subsequently ironed drink can the bottom has still the thickness (about 0.30 mm) of the original material, while the wall is thinned to 0.10 mm. In this case, near the free end of this ironed can a larger wall-thickness is preserved, e.g. about 0.16 mm, in order to make easier the production of flanged edges for the fastening of the top.
If the shell of a three-piece can is to be given a wall-thickness comparable with that of a two-piece can, there remain considerable problems to be solved in the rolling of such super-thin plate of about 0.10 mm thickness, and during further processing. This thin material is very prone to kinking, so that extra care has to be given when transporting it, while during cutting of the plate not only is there a transport problem but the prevention or remedying of burr formation must be achieved. Consequently, extra care is necessary in order to shape this extra thin material into a cylinder and then to weld it. If soldering or cementing is used, this body has a disadvantage with respect to the two-piece ironed can, since the solder or cement material may make the can less suitable for various food stuffs and/or drinks. Special problems may also have to be solved when forming flanged edges from these extra thin-walled cylinders and subsequently fastening the bottom and lid to them, while achieving an adequate seal.
The latter problems are solved with two-piece ironed cans by the possibility during the ironing process of giving the material a greater thickness near the free end of the can, but this is not possible with the body of a three-piece can made from rolled material.
Nevertheless it appears that for certain applications three-piece cans still have advantages over two-piece cans, so that there is still a clear need for the development of a satisfactory method of making a thin-walled three-piece body.
It is mentioned here that U.S. Pat. No. 3,428,010 discloses a deep-drawn and ironed can which is made as a three piece can by cutting off and shaping the closed end of the drawn and ironed tube to provide the can top. The can bottom is provided by a conventionally flanged-on disc. This process involves a large number of steps.
It may be noted further that with two-piece cans there is a need to cut the end of the formed shell accurately to size (trimming), and at the same time the smoothest possible cutedge is required to be suitable as a flanged edge. With an ironed can this implies wastage at one end. This wastage is expensive.